The present invention relates, in general, to heat exchanger tubes and, more particularly, to elongated elliptical, oval or flat heat exchanger tubes and their construction.
Some conventional heat exchangers typically comprise tubes having circular cross-sections and integrally bonded cooling fins. More recently, new heat exchanger designs have been developed using elliptical or flat heat exchanger tubes. These tubes are shaped similar to an airfoil and have surface bonded, peripheral cooling fins oriented in-line with the direction of air flow. Because these advanced heat exchanger tubes have configurations consisting of thin-walled elliptical cross-sections with major to minor axis ratios sometimes greater than 10, excessive deflections/deformations of the flat side walls due to external differential pressures of up to 15 psi have been observed, particularly in the central region. Such large deflections can cause cyclic fatigue, resulting in bond failure at the tube/cooling fin interface. An economical method of reducing or eliminating the flat tube wall deflection has thus been found necessary to enable the commercial manufacture of these advanced heat exchanger systems.
There are numerous granted U.S. patents drawn to designs of the aforementioned elliptical tube heat exchangers. However, none of them provide any type of internal stiffening to prevent the mentioned deflection problems. Any type of internal structure found in these patents which could be construed as adding stiffness to the elliptical heat exchanger tube is formed to produce separate internal passages within the elliptical heat exchange tube. These separate internal passages provided in the heat exchanger tubes are maintained separate and are not fluidically inter-connected, at least along the length of the tube.
Among these discussed prior art references are found the following U.S. patents which add structure which subdivides the elliptical tubes into chambers approximating circular tubes more than elliptical tubes with a major to minor axis ratio in excess of 10.
Haussmann (U.S. Pat. No. 5,251,692) discloses a flat tube heat exchanger having headers and a number of flat tubes between the headers. The flat tubes have flat sides and rounded short sides, as well as internal reinforcing ribs. The reinforcing ribs are spaced apart from one another by a distance ranging from about one to about two times the distance D between the outer surfaces of the flat tube 12.
Hughes et al. (U.S. Pat. No. 5,279,360) discloses an evaporator having tubes with a major and minor axis and containing therein a plurality of flow passages of generally triangular configuration. The flow passages are separated by integral webs extending between the sides of the tube. The webs serve to define individual and discrete flow paths, and strengthen the tubes against buckling of one side wall toward or away from the other when a bending force is applied across the tube major dimension.
Sasaki (U.S. Pat. No. 5,318,114) is drawn to a multi-layered type heat exchanger which includes a plurality of substantially parallel flat tubes. Each flat tube includes a partition wall dividing its interior into two fluid passages.
Grieb et al. (U.S. Pat. No. 4,766,953) is drawn to a shaped tube with an elliptical cross-section and a multi-chambered design for tubular heat exchangers. At least two cross rows pass through an interior space of the tube at a distance from one another. The tube is made by bending an endless metal strip into two semi-finished products with congruent profiles, each having the shape of an isosceles triangle with rounded vertices and an elongated leg. The semi-finished products are placed against one another so that the free end of the elongated leg of one semi-finished product abuts the triangle base edge of the other semi-finished product.
Kritzer (U.S. Pat. No. 4,360,958) is drawn to a method of making multi-port heat exchangers when the tubular members are made of a metal that does not lend itself well to being extruded into a plurality of passageways. Multiple passageways are provided in the tube however, by dividers inserted and adhered thereinto.
Modine (U.S. Pat. No. 2,396,522) is drawn to a radiator tube construction wherein upper and lower flat sheets are separated from one another and divided into a plurality of compartments by various members, some of which are circular while others have square cross-sections. These interspersed members are referred at various locations as being wire or the like.
Yokoyama et al. (U.S. Pat. No. 5,203,403) is drawn to a plate fin heat exchanger, and particularly to the cylindrical fin collars themselves. Side ridge portions promote increased turbulence and heat transfer efficiency.
U.S. Pat. Nos. 5,186,250 and 5,186,251 to Ouchi et al, and Joshi, respectively, disclose tubes for heat exchangers and methods for manufacturing same. In the '250 patent the tube is a flat tube comprising a pair of plane walls separated a distance from one another by U-shaped bent portions of the walls themselves. Alternatively, the U-shaped portions can comprise dimples 16. The '251 patent shows a heat exchanger with double row tubes made by a roll forming operation from a single piece blank that has a centralized vertical connector web of the thickness of the blank that connects and supports opposite side walls of the tube to augment tube burst strength for high internal pressures. The vertical connector web also effectively eliminates tube crushing from compression loads when inserted onto a core of tubes.
Thus it is seen that an effective stiffener for elliptical, oval or flat heat exchanger tubes having a ratio of major to minor axis of 10 or larger was needed which would allow the flow of fluid across the tube stiffeners.